High-bandwidth technologies which are prevalent today are frequently implemented on existing copper wire infrastructure that was intended for plain old telephones (POTS) communication. One such technology is Digital Subscriber Line (DSL). DSL comes in multiple variations such as ADSL, HDSL, IDSL, SDSL, RADSL and VDSL collectively known as xDSL. Asymmetric digital subscriber line (ADSL) allows users a higher data rate downstream (i.e. to the customer) than upstream (i.e. the service provider).
These high-bandwidth systems use multi-carrier modulation schemes such as Carrier-less Amplitude and Phase modulation (CAP) and Discrete Multi-tone (DMT) for wired media communication. The advantage of such schemes is that they are suited for high-bandwidth applications of 2 Mbps or higher upstream (subscriber to provider) and up to 8 Mbps downstream (provider to subscriber). DMT systems divide the spectrum above the 4-KHz voice frequency band into 256 narrow channels called sub-channels (sometimes referred to as DMT tones or bins. Each sub-channel is 4.3125 KHz wide. Quadrature amplitude modulation (QAM) is used for each sub-channel. Different numbers of bits may be allocated to different sub-channels, and sub-channels with a larger signal-to-noise ratio (SNR) typically carry more data while other sub-channels with a smaller SNR carry relatively less data. The process of determining which sub-channel should carry more or less data based on the SNR is termed bit-loading, bit-allocation, or bit-stuffing.
The Shannon formula for calculating the bit rate B for a QAM system in unit bandwidth is representation equation 1:B=log2(1+coding_gain*SNR/snr_gap)  (1)where, SNR is the signal-to-noise ratio, coding_gain is the coding gain, snr_gap is the quantity used to determine the efficiency of a modulation or encoding scheme as compared to the ideal scheme.
DMT is a combination of many QAM sub-systems i, where i is an integer between 1 and the maximum. After the signals are transmitted through the channel, the signals are attenuated differently, thereby creating a different SNR_i for different sub-systems. After summing equations (1), N times for each sub-carrier a rate that can support a DMT system is achieved.
There are many different prior art methods for allocating bits to sub-channels (bins). In these algorithms, a common characteristic is that an equal error rate criterion and a single value of SNR-GAP is used to load bits in different sub-channels or “bins.” One of those methods uses different signal-to-noise ratio (SNR) for different sub-systems and considers the influence of SNR GAP and coding gain. This method creates a pre-calculated look-up table (saved in memory) to compare the actual bit rate, and then a decision is made as to the number of bits the system can support. Such a method is disclosed in U.S. Pat. No. 6,516,027 to Kapoor et al. hereby incorporated by reference it its entirety. In addition, this method uses a symbol error rate for the bit-loading, instead of a bit error rate.
Another known bit-loading method determines a preliminary bit assignment for each sub-channel by optimizing a DMT system with N carriers for transmission over an additive white Gaussian channel, given an available bandwidth and assuming that all sub-channels are turned on. The sub-channels that receive negative bit assignments are excluded, and the procedure for determining the bit assignments is repeated until all the bit assignments are positive for the remaining turned-on sub-channels. Then, for a given bit error rate, the method arranges the power and data rate to optimize the DMT transmission bandwidth. Such a method is disclosed in U.S. Pat. No. 6,704,367 to Wang et al. hereby incorporated by reference in its entirety.
The two main requirements for implementing any bit-loading algorithm are simplicity and precision. For an un-coded, zero-margin system with a given bit, error rate (BER), the SNR GAP Γ employed in the calculation of bits to be allocated to each sub-channel, is fixed to a given value in the existing algorithms. As shown in FIG. 1, a bit allocation method begins in step 110 with calculating the signal-to-noise ratio for a sub-channel. Then, in step 120, a common coding gain is added to the SNR. Next, in step 130, a common signal-to-noise (SNR) ratio gap is subtracted. Then, with the calculated information, the bits to be allocated to that sub-channel are calculated in step 140. Those steps are repeated for all the available DMT sub-channels, and the calculated bits to be allocated are added together in step 150, in order to achieve the total number of bits that can be allocated to the system as a whole in step 160. However, the bit rate calculated using a fixed SNR GAP and a fixed Trellis coding gain is inaccurate and cannot reflect the boundary effect, which is the difference between symbol error multiplicity, the bit error multiplicity and the constellation size. Other drawbacks also exist in current bit-loading methods. Therefore, there is a need for a method and system for attaining an improved bit-loading in discrete multi-tone modulation (DMT) data communication systems.